1. Field of the Invention
The present invention is broadly concerned with an extrusion apparatus and method designed to efficiently handle and extrude material heretofore presenting extreme processing problems such as wood chips, sawdust and other woody residues, municipal solid waste, and crop residues. More particularly, it is concerned with such a device and method which makes use of a transition screw section within the barrel which serves to even out material flow and assist in buildup of relatively high pressures and temperatures; moreover, the overall extrusion apparatus preferably includes an adjustable die which further facilitates the extrusion operation.
2. Description of the Prior Art
A number of processes and devices have been proposed in the past for the purpose of restructuring cellulose- or fiber-bearing materials to produce a comminuted or defibrated product. To give but one example, defibrated wood is used in the production of so-called fiber board. The defibrated wood can be produced by a technique known as "gun puffing" wherein wood chips are treated with high pressure steam in a large vessel followed by a sudden release of pressure to separate and restructure the wood chips into fibers and thus obtain the defibrated product. Another type of process to achieve this result involves disc refining of wood chips wherein, after an initial pressure treatment, the wood chips are conveyed to a large multiple disc refiner which is likewise operated under pressure. Large commercial units of this type are capable of refining wood chips down to essentially single fiber size, but for this purpose require a very large motor, e.g., two thousand horsepower. One significant problem with disc refiners is maintenance and parts replacement costs. These units are prone to wear out critical operating components at a rather considerable rate, and are therefore deficient.
Treatment of municipal solid waste has also been practiced in the past in order to render this material uniform and usable as a fuel. One technique for treatment of municipal solid waste is known as hydropulping. In this process, a slurry of the waste and water is introduced into a large kettle having a perforated bottom. A rotating scraper blade is situated adjacent the perforated bottom, and serves to reduce and restructure the solids passing through the kettle bottom.
Generally speaking, all of the processes mentioned above are deficient in one important respect, i.e., they consume large amounts of energy. For example, the disc refining process for wood chips, although effective for producing the desired defibrated wood, can be almost prohibitively expensive from an energy requirement standpoint. This stems not only from the inherent electrical power consumption of the large driving motor and steam used in treatment of woodchips, but also from the fact that large volumes of water must be pumped and handled along with wood chips.
Attempts have been made in the past to extrusion process wood chips or other cellulose- or fiber-bearing materials. Theoretically, the advantages of extrusion processing of these materials are manifest, particularly from the standpoint of energy consumption. However, severe problems are encountered in practice, stemming principally from the great difficulty in smoothly and efficiently extruding rough, coarse materials such as wood chips. Typically, attempts to extrude wood chips result either in excessive surging and blowing or other signs of uneven operation, or perhaps more often complete plugging of the machine, necessitating a break down and cleanup of the machine. In view of these problems, it is believed that no commercially successful extrusion apparatus or process has heretofore been developed which can effectively restructure and defibrate cellulose- or fiber-bearing materials.